The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to an improved RF volume coil which can be decoupled from other coils in the system during an NMR pulse sequence.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .UPSILON. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
In an NMR imaging system the excitation field is in the radio frequencies and is produced by an RF coil. The NMR signal which results is also in the radio frequencies and may be sensed by the same RF coil or a separate RF coil. For example, it is common practice to employ an RF volume coil which produces a uniform RF excitation field throughout a large volume and to employ a separate RF surface coil to receive the resulting NMR signal from a well defined region within the large volume.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The magnetic field gradients used for imaging are short in duration (milliseconds) and high in field strength. During a single NMR measurement pulse sequence for example, the gradient for each axis (G.sub.x, G.sub.y, G.sub.z) may be pulsed one or more times and reversed in polarity. The resulting magnetic fields produce forces on the electrically conductive elements of the system, including the RF coil. These forces cause movement or vibration of these elements and such movement of the RF coil elements can induce noise into the acquired NMR data. The physical construction of the RF coil is, therefore, an important design consideration.
As noted above, the RF excitation and the resulting NMR signal are both at the Larmor frequency. As a consequence, both the RF excitation coil and the NMR receive coil must be tuned to resonate at this frequency. The cross coupling of two coils tuned to the same frequency and located within the same region of interest presents a problem. This cross coupling causes nonuniformity in the RF excitation field, frequency shifts and impedance shifts in both coils, and the acquisition of a degraded NMR signal.
The solution to the cross coupling problem is to detune the receive coil while the RF excitation field is being produced and to detune the RF excitation coil when the NMR signal is being acquired. Mechanical switches are not fast enough to use during the NMR pulse sequence and electronic switches must be used. For example, the use of diodes as switches to selectively detune a pair of co-planar surface coils is described in U.S. Pat. No. 4,620,155. Similarly, a set of diodes are employed as switches in U.S. Pat. No. 4,833,409 to detune a whole body coil by shorting its end rings to a surrounding shield. While the latter solution effectively detunes the whole body coil during acquisition of the NMR signal, its performance is less than ideal. First, the increased RF losses in this circuit and connections to the shield, as well as the leakage impedance of the diodes degrades the quality factor of the coil and reduces image quality. And second, the physical connections between the RF coil and the surrounding shield are costly to manufacture if they are to withstand the physical beating imposed by the forces caused by the high gradient field pulses used in modern NMR pulse sequences.